ABSTRACT An effective HIV vaccine is essential to achieve a durable end to the HIV/AIDS pandemic. Preferential HIV infection of activated human CD4 T cells, which are usually induced by vaccination, has posed a unique challenge for the development of a safe and protective HIV vaccine. A gap in knowledge is whether or not vaccine-induced CD4 T cells are preferential HIV targets in vaccination. Therefore, understanding the susceptibility of vaccine-generated CD4 T cells to HIV and its potential impact on vaccine outcomes is critical and will provide novel insights into rational HIV vaccine design. Viral vectors are central to HIV vaccine development. To date, a number of HIV vaccine vectors (e.g. poxviral vector ALVAC and adenoviral vector Ad5) have been tested in clinical trials but were unexpectedly associated with distinct vaccine outcomes. Currently, another knowledge gap is that the host response to different viral vectors following HIV vaccination remains poorly understood. Given the dual roles of CD4 T cells in the generation of protective immunity during HIV vaccination, we speculate that a successful HIV vaccine approach, especially those utilizing viral vectors, must induce vector-reactive CD4 T cells that are resistant or less susceptible to HIV. As such, our recent studies have shown that CD4 T cells induced by different vectors (ALVAC and Ad5) manifest distinct susceptibility to HIV, which is associated with the phenotypes of vector- induced T cells as well as with the innate signals (inflammasome activation) primed by these two vectors. Human adenovirus 26 (Ad26) vector has become increasingly important for HIV vaccine development and is under intensive clinical testing. Due to the failure of Ad5 vector and the concern that Ad5 vaccination may be associated with enhanced HIV infection in some vaccine recipients, it is critical to understand whether or not, and how, HIV preferentially infects Ad26 vector-induced CD4 T cells. We hypothesize that Ad26 vector induces vector-reactive CD4 T cells that are distinct from those induced by ALVAC in phenotype and HIV susceptibility, which contributes to the differential vaccine outcomes. In this R21 application, we will collaborate with IAVI (International AIDS Vaccine Initiatives), HVTN (HIV Vaccine Trial Network), MHRP (Military HIV Research Program), and Genoveffa Franchini, to examine in vitro and in vivo susceptibility of Ad26 vector- induced CD4 T cells to HIV/SIV in vaccinated human individuals (Aim 1) and non-human primates (Aim 2) as compared to those induced by ALVAC. We will also define the underlying innate immune mechanisms, especially effects on the inflammasome pathway. We expect that this R21 project will be highly impactful since it presents an effort to systematically investigate HIV/SIV susceptibility of CD4 T cells induced by two clinically important vectors in parallel. Our long-term goal is to discover novel approaches (e.g. adjuvants) based on the identified immune parameters to enhance HIV resistance of vaccine-induced CD4 T cells in HIV vaccination.